Backlight unit equipped with light emitting diodes

ABSTRACT

Disclosed herein is a backlight unit equipped with LEDs. The backlight includes an insulating substrate, a plurality of LED packages, an upper heat dissipation plate, and a lower heat dissipation plate. The insulating substrate is provided with predetermined circuit patterns. The LED packages are mounted above the insulating substrate, and are electrically connected to the circuit patterns. The upper heat dissipation plate is formed on the insulating substrate, and is configured to come into contact with the circuit patterns and to dissipate heat. The lower heat dissipation plate is formed on the insulating substrate, and is configured to transmit heat transmitted through the upper heat dissipation plate. The upper heat dissipation plate and the lower heat dissipation plate are connected to each other by at least one through hole, and the through hole and the upper heat dissipation plate have a predetermined area ratio.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Divisional of U.S. application Ser. No.11/714,193, filed on Mar. 6, 2007 now U.S. Pat. No. 7,795,635, claimingpriority of Korean Patent Application No. 10-2006-0021010, filed on Mar.6, 2006, the entire contents of each of which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a backlight unit installed ina Liquid Crystal Display (LCD) and, more particularly, to a backlightunit that is capable of effectively dissipating the large quantity ofheat that is generated by Light-Emitting Diodes (LEDs), which are lightsources.

2. Description of the Related Art

A backlight unit is a device that is installed in an LCD using theprinciple in which liquid crystals change their molecular arrangementaccording to applied voltage, and provides light and illuminates ascreen from behind. Although such backlight units were mainly formed ofcold-cathode tubes, LEDs have attracted attention as backlight units dueto the advantages with respect to life span, brightness, colorreproducibility, etc.

When LEDs are used as light sources, LEDs require substrates, unlikecold-cathode tubes. Since LEDs emit large quantities of heat whileradiating light, metal core substrates (metal core printed circuitboards) having an excellent heat dissipation characteristic have beenused. Although metal core substrates have an excellent heat dissipationcharacteristic, they are very expensive. Accordingly, the high cost ofthe metal core substrates is one of the principal factors that impedethe cost competitiveness of the backlight units formed of the metal coresubstrates. As a result, there is a trend toward the use of relativelyinexpensive epoxy resin insulating substrates. An example of aconventional backlight unit in which LEDs are mounted on such aninsulating substrate is illustrated in FIG. 4.

As illustrated in FIG. 4, a backlight unit 200 includes an insulatingsubstrate 210, a plurality of LED packages 230 and a chassis 250.

Circuit patterns 211 and 212 are formed on the insulating substrate 210by coating an epoxy resin FR4-core with a copper foil and etching thecopper foil.

Each of the LED packages 230 is mounted such that an LED chip 231 isdirectly connected to one LED electrode 232 and is wire-bonded to theother LED electrode 233.

The LED chip 231 and the LED electrodes 232 and 233 are placed within aplastic mold casing 234, and the casing 234 is covered with an epoxyresin lens 235.

The LED package 230 is mounted on the insulating substrate 210, and iselectrically connected to positive and negative electrodes, that is, thecircuit patterns 211 and 212.

The chassis 250 is made of material having excellent thermalconductivity, such as metal, and is placed below the insulatingsubstrate 210 with a heat pad 270 placed therebetween so as to provideelectrical insulation and decrease contact thermal resistance.

In the conventional backlight unit 200, a thermal resistance obtainedusing the equation R=L/KA is about 63.5 K/W, which is very high, whenthe thickness L of a substrate is 0.8 mm, the thermal conductivity K ofthe substrate is 0.35, and the area A of the substrate is 36 mm².

As a result, the conventional backlight unit having the above-describedconstruction exhibits the very poor thermal conductivity of theinsulating substrate, therefore it is difficult to effectively eliminateheat generated by the LED chips, with the result that the temperature ofthe LED chips continuously increases. Accordingly, the amount of lightemitted by the LED chips decreases, variation in wavelength occurs, andthe reliability of the LED chips decreases, thus resulting in a reducedlifespan.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made keeping in mind theabove problems occurring in the prior art, and an object of the presentinvention is to provide a backlight unit, which employs an inexpensiveinsulating substrate as a substrate and can effectively dissipate thelarge quantity of heat generated in LED chips, which are light sources.

In order to accomplish the above object, the present invention providesa backlight unit equipped with LEDs, including an insulating substrateprovided with predetermined circuit patterns on one surface thereof; aplurality of LED packages mounted above the insulating substrate andelectrically connected to the circuit patterns; an upper heatdissipation plate formed on the insulating substrate, and configured tocome into contact with the circuit patterns and to dissipate heatgenerated in the LED packages; and a lower heat dissipation plate formedon a remaining surface of the insulating substrate, and configured totransmit heat transmitted through the upper heat dissipation plate;wherein the upper heat dissipation plate and the lower heat dissipationplate are connected to each other by at least one through hole thatpasses through the insulating substrate and is plated with platingmaterial on an inner wall thereof, and the through hole and the upperheat dissipation plate have a predetermined area ratio.

The upper heat dissipation plate and the lower heat dissipation platemay be made of copper and each have a thickness of about 0.35 μm.

The area ratio of the through hole and the upper heat dissipation platemay range from 1:10 to 1:300.

The insulating substrate may be provided with an insulating film, andthe insulating film may be configured to cover the lower heatdissipation plate and may be applied through sputtering or spraying.

The insulating film may have a thickness of about 0.35 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will be more clearly understood from the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a schematic diagram showing a backlight unit equipped withLEDs according to a preferred embodiment of the present invention;

FIG. 2 is an enlarged partial plan view showing the principal parts ofthe backlight unit of FIG. 1;

FIG. 3 is a schematic partial sectional view taken along line A-A ofFIG. 2; and

FIG. 4 is a schematic partial sectional view showing a conventionalbacklight unit equipped with LEDs.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference now should be made to the drawings, in which the samereference numerals are used throughout the different drawings todesignate the same or similar components.

With reference to the accompanying drawings, a backlight unit equippedwith LEDs according to a preferred embodiment of the present inventionis described in detail below.

As shown in FIG. 1, a backlight unit 100 according to the presentembodiment includes a backlight module which includes a substrate 110and a plurality of LED packages 130, and a chassis 150 on which thebacklight module is fixedly mounted.

The substrate 110 is formed by coating an epoxy resin FR4-core, that is,an insulating substrate, with a copper foil and forming circuit patterns(not shown) through an to etching process. Furthermore, the substrate110 is thinly coated with insulating material on the bottom surfacethereof, thereby forming an insulating film 113, different from aconventional heat pad.

Since an insulating substrate other than a metal substrate is used asthe substrate 110, the cost of the backlight unit 100 is reduced, andthe cost of the LCD, on which the backlight unit 100 is mounted, canalso be reduced.

The LED packages 130 form a plurality of sets of LED packages foremitting red light, green light and blue light, and the plurality ofsets of LED packages are mounted on the substrate 110 in accordance withdesign.

The chassis 150 is a part that forms the casing of the backlight unit100, and is made of metallic material having excellent thermalconductivity, such as aluminum or copper. It is preferred that thechassis 150 be made of aluminum material so as to reduce the weightthereof.

The substrate 110 and the chassis 150 are fixedly combined with eachother using a fastening means, such as bolts 120 that pass throughthrough holes 115 and 155 formed through the corners of the substrate110.

With reference to FIGS. 2 and 3, which are enlarged views of theprincipal parts of the backlight unit 100, the backlight unit 100 isdescribed in detail.

The backlight unit 100 shown in FIGS. 2 and 3 includes a substrate 110,a plurality of LED packages 130 and a chassis 150.

The substrate 110 is an insulating substrate, and is provided withcircuit patterns 111 and 112 on the upper surface thereof using a copperfoil. The insulating film 113 is formed by coating the substrate 110with insulating material on the bottom surface thereof. In this case, acoating method may be one of the various methods, such as sputtering andspraying.

Furthermore, upper and lower heat dissipation plates 114 and 116 arerespectively formed on the upper and lower surfaces of the substrate 110so as to dissipate heat generated in the LED packages 130. Inparticular, the upper heat dissipation plate 114 comes into directcontact with the circuit pattern 111.

Since, in the present embodiment, the insulating material used as theinsulating film 113 can be formed to have thermal conductivity lowerthan that of a conventional heat pad and a very thin thickness of, forexample, 0.35 μm, it can achieve low thermal resistance.

As an example, a conventional heat pad having a thickness of about 0.2mm has a heat resistance of about 2*10⁻⁴ W/K per unit area, whereas theinsulating film 113 of the present embodiment has a thermal resistanceof about 5.8*10⁻⁵ W/K, which corresponds to a thermal resistancereduction of about 71%.

Two through holes 115 are formed through the substrate 110 and the upperand lower heat dissipation plates 114 and 116 perpendicular to thesubstrate 110. The two through holes 115 are plated with copper on theinner walls thereof, therefore the upper and lower heat dissipationplates 114 and 116 formed on the upper and lower surfaces of thesubstrate 110 are connected to each other. As a result, heat generatedin the LED packages 130 can be conducted through the upper heatdissipation plate 114 to the lower heat dissipation plate 116 and canthen be dissipated to the chassis 150.

Since the upper and lower heat dissipation plates 114 and 116 are madeof material having excellent thermal conductivity such as copper, theeffect of heat dissipation increases in proportion to the areas thereof.It is preferable to design the areas of the upper and lower heatdissipation plates 114 and 116 to be as large as possible inconsideration of other external conditions, that is, the number of LEDsand the overall area of the substrate.

Meanwhile, since the through holes 115 are only plated with copper onthe inner walls thereof, but are not filled with filling material, theweight and cost of the backlight unit 100 can be reduced.

In this case, as the number of through holes 115 increases, the amountof heat conducted from the upper heat dissipation plate 114 to the lowerheat dissipation plate 116 increases, thus resulting in an increase inthe amount of heat dissipated through conduction. However, in the casewhere a large number of through holes 115 is formed, the area of theupper heat dissipation plate 114 is decreased, thus resulting in adecrease in the amount of heat dissipated through convection.

Accordingly, it is preferable to form the through holes 115 so that theamount of heat dissipated through conduction and the amount of heatdissipated through convection can be maximized. For this purpose, in thepresent embodiment, the through holes 115 and the upper heat dissipationplate 114 are formed such that the ratio of the areas thereof is about1:129. However, it is preferred that the ratio be within a range of 1:10to 1:300.

Each of the LED packages 130 includes an LED chip 131, LED electrodes132 and 133, a plastic mold casing 134 and a lens 135.

The LED chip 131 is a means for emitting red light, green light or bluelight, and is directly mounted on one LED electrode 132 and iselectrically connected to another LED electrode 133 through wirebonding.

The LED electrode 132 is mounted on the positive one 111 of the circuitpatterns 111 and 112 formed on the substrate 110, whereas the LEDelectrode 133 is mounted on the negative one 112 of the circuit patterns111 and 112.

The LED chip 131 and the LED electrodes 132 and 133 are protected fromthe outside by a plastic mold casing 134, and the casing 134 is coveredwith the epoxy resin transparent lens 135.

The chassis 150 is made of material having excellent thermalconductivity, such as metal, and is placed under the insulatingsubstrate 110.

In the present embodiment, the thickness of the substrate 110 is about0.8 mm, each to of the thicknesses of the upper and lower heatdissipation plates 114 and 116 and the insulating film 113 is about 0.35μm, the area of the insulating film 113 is about 36 mm², and thediameter of the through holes 115 is about 0.3 mm.

When thermal resistance is calculated based on the above-describeddimensions,

$\begin{matrix}{R = \{ {{1/R_{air}} + {1/R_{copper}} + {1/R_{substrate}}} \}^{- 1}} \\{= \{ {( {{KA}/L} )_{air} + ( {{KA}/L} )_{copper} + ( {{KA}/L} )_{substrate}} \}^{- 1}} \\{= \begin{Bmatrix}{{( {0.03*0.0831*10^{- 6}} )/( {1.8*10^{- 3}} )} +} \\{{( {401*0.0582*10^{- 6}} )/( {0.8*10^{- 3}} )} +} \\{( {0.35*35.8587*10^{- 6}} )/( {0.8*10^{- 3}} )}\end{Bmatrix}^{- 1}} \\{= {22.3\mspace{14mu} K\text{/}W}}\end{matrix}$

This value is a value that corresponds to about 35% of a thermalresistance of 63.5 K/W, which is obtained using the conventionaltechnology when the same substrate is used, from which it can be seenthat the thermal resistance is reduced by 65%.

According to the present invention, the cost of the backlight unit canbe reduced because the insulating substrate is used as the substratethereof, and the luminance and lifespan of LEDs can be increased becauseheat can be effectively dissipated using the heat dissipation plates.

Furthermore, a very thin insulating film, instead of a conventional heatpad disposed between the substrate and the chassis, is employed,therefore thermal resistance is considerably reduced, therebyconsiderably increasing the amount of heat dissipation.

Furthermore, the area of the heat dissipation plate and the area of thethrough holes are determined at an appropriate ratio so that thedissipation of heat generated by the LEDs can be optimized through theconvection and conduction of the heat dissipation plate, therebymaximizing the dissipation of heat.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A backlight unit equipped with Light-Emitting Diodes (LEDs),comprising: an insulating substrate provided with predetermined circuitpatterns on one surface thereof; a plurality of LED packages mountedabove the insulating substrate and electrically connected to the circuitpatterns; an upper heat dissipation plate formed on the insulatingsubstrate, and configured to come into contact with the circuit patternsand to dissipate heat generated in the LED packages, the upper heatdissipation plate has rectangular cross section; a lower heatdissipation plate formed on a remaining surface of the insulatingsubstrate, and configured to transmit heat transmitted through the upperheat dissipation plate; and a chassis disposed in the remaining surfaceof the insulating substrate and fixedly combined with the insulatingsubstrate, wherein the upper heat dissipation plate and the lower heatdissipation plate are connected to each other by at least one throughhole that passes through the insulating substrate and is plated withplating material on an inner wall thereof, and the through hole isdirectly connected to the upper heat dissipation plate and the lowerheat dissipation plate, wherein the LED packages comprise an LED chip,the LED chip is not disposed on the upper heat dissipation plate.
 2. Thebacklight unit as set forth in claim 1, wherein the upper heatdissipation plate and the lower heat dissipation plate are made ofcopper and each have a thickness of about 0.35 μm.
 3. The backlight unitas set forth in claim 1, wherein the area ratio of the through hole andthe upper heat dissipation plate ranges from 1:10 to 1:300.
 4. Thebacklight unit as set forth in claim 1, wherein the insulating substrateis provided with an insulating film, and the insulating film isconfigured to cover the lower heat dissipation plate.
 5. The backlightunit as set forth in claim 4, wherein the insulating film has athickness of about 0.35 μm.
 6. The backlight unit as set forth in claim1, wherein the insulating substrate is provided with the circuitpatterns on an epoxy resin.
 7. The backlight unit as set forth in claim1, wherein the LED packages comprising: two LED electrodes connected tothe circuit patterns; an LED chip mounted on one of the LED electrodes;a plastic mold casing protecting the LED electrode and the LED chip fromthe outside; and a lens covering the plastic mold casing.
 8. Thebacklight unit as set forth in claim 1, wherein the chassis is made ofmetallic material having thermal conductivity.